1. Field of the Invention
The invention relates to flash memory devices, more particularly to systems and methods of managing memory addresses in a large capacity multi-level cell (MLC) based flash memory device housed in a tubular casing.
2. Description of the Related Art
As flash memory technology becomes more advanced, flash memory is replacing traditional magnetic disks as storage media for mobile systems. Flash memory has significant advantages over floppy disks or magnetic hard disks such as having high-G resistance and low power dissipation. Because of the smaller physical size of flash memory, they are also more conducive to mobile systems. Accordingly, the flash memory trend has been growing because of its compatibility with mobile systems and low-power feature. However, advances in flash technology have created a greater variety of flash memory device types that vary for reasons of performance, cost and capacity. As such, a problem arises when mobile systems that are designed for one type of flash memory are constructed using another, incompatible type of flash memory.
New generation personal computer (PC) card technologies have been developed that combine flash memory with architecture that is compatible with the Universal Serial Bus (USB) standard. This has further fueled the flash memory trend because the USB standard is easy to implement and is popular with PC users. In addition, flash memory is replacing floppy disks because flash memory provides higher storage capacity and faster access speeds than floppy drives.
In addition to the limitations introduced by the USB standard, there are inherent limitations with flash memory. First, flash memory sectors that have already been programmed must be erased before being reprogrammed. Also, flash memory sectors have a limited life span; i.e., they can be erased only a limited number of times before failure. Accordingly, flash memory access is slow due to the erase-before-write nature and ongoing erasing will damage the flash memory sectors over time.
To address the speed problems with USB-standard flash memory, hardware and firmware utilize existing small computer systems interface (SCSI) protocols so that flash memory can function as mass-storage devices similarly to magnetic hard disks. SCSI protocols have been used in USB-standard mass-storage devices long before flash memory devices have been widely adopted as storage media. Accordingly, the USB standard has incorporated traditional SCSI protocols to manage flash memory.
As the demands for larger capacity storage increase, the flash memory device needs to keep up. Instead of using single-level cell flash memory, which stores one-bit of information per cell, multi-level cell (MLC) flash memory, or hybrid flash memory, which is assembled partially SLC and partially MLC, is used. The MLC flash memory allows at least two bits per cell. However, there are a number of problems associated with the MLC flash memory. First, the MLC flash memory has a low reliability. Secondly, the MLC flash memory data programming rules require writing to an ascending page in the same block or writing to a blank new page if there are data existed in the original page. Finally, a larger capacity requires a large logical-to-physical address look up table. In the prior art approach, the size look up table is in direct portion with the capacity of the flash memory. This creates a huge problem not only to the cost, but also to the physical size of the flash memory device. Furthermore, the traditional usage of the flash memory devices is generally in a very clean and relatively mild environment, thus the packaging design such as enclosure of the flash memory device is not suitable for hostile environment such as military and heavy industrial applications.
Modern portable computer peripheral devices for storing confidential data take many mechanical forms. In most cases, such peripheral devices have been reduced to “pocket size”, meaning that they can literally be carried in a user's pocket in the same manner as a wallet or set of keys. One example of particular interest is a pen-type flash device having a USB connector plug that can be connected to a USB port of a standard computer. The USB plug connector is protected by a removable cap when not in use. A problem with convention pen-type peripheral devices is that the removable cap can become inadvertently lost while the device is in use, thereby leaving the USB plug connector exposed to damage or contamination.
An alternative to conventional pen-type peripheral devices is a “press-push” memory device, which provides a connector that retracts into a housing of the memory device for protection when not in use. A device with a retractable connector generally has a button feature on the outside of its housing that allows a user to manually slide the connector between a retracted position and an extended (deployed) position. In the extended position, the connector extends through an opening in the housing so that it may be plugged into a receptacle. In the retracted position, the connector is contained within the housing and is protected by the housing, thereby obviating the need for a separate cap that can be lost.
Although “press-push” memory devices avoid the problems of conventional pen-type peripheral devices, e.g., by avoiding the need for a separate cap, the molded plastic housing typically used for these devices can be easily crushed when the device is accidentally dropped or subjected to a blunt impact force, leading to undesirable resistance or jamming that prevents the desired retraction of the connector when not in use.
To address the strength/durability issues associated with conventional all-plastic “press-push” memory devices, many recently produced “press-push” memory devices are made with durable metal tubular casings. The metal tubular casing houses a plastic “press-push” mechanism that supports the PCBA and is movable inside the metal tubular casing to deploy or retract a USB plug connector. In addition to providing durability, metal tubular casings also help with heat dissipation, which is particularly important in newer USB 3.0 flash drives that can consume more power (generates more heat) than USB 2.0 flash drives (the current usage for USB 2.0 is rated up to 500 mA compared to 950 mA of USB 3.0; therefore, the maximum power consumption of USB 2.0 is 2.5 W (500 mA×5V) compared to 4.75 W of USB 3.0). The bus power 5V is supplied by the USB computer host.
Although using metal tubular casings to produce durable “press-push” memory devices addresses the durability and heat dissipation issues associated with conventional all-plastic “press-push” memory devices, the metal tubular casings introduce a new set of problems.
One problem with “press-push” memory devices made with metal tubular casing is that the use of different housing materials (i.e., metal and plastic) complicates the manufacturing process due to the different manufacturing techniques that are required, which can result in mismatched structures that fail to connect properly, leading to defective devices that fall apart prematurely fail to operate properly, leading to undesirable resistance or jamming that prevents the desired retraction of the connector when not in use.
Another problem associated with such “press-push” memory devices is that the metal tubular casing is much harder than the plastic “press-push” mechanism, causing accelerated wear of the plastic mechanism, leading to undesirable resistance or jamming that prevents the desired retraction of the connector when not in use.
Therefore, it would be desirable to have improved methods and systems of managing memory addresses in a large capacity multi-level cell (MLC) flash memory device. What is also needed is a retractable portable computer peripheral apparatus for housing a large capacity multi-level cell (MLC) flash memory device that overcomes the problems associated with conventional press-push memory devices housed in metal tubular casings.